Noise reduction system



July 18, 1939. E. o SELBY 2,166,694

NOISE REDUCTION SYS TEM Filed Nov. 19, 1936 2 Sheets-Sheet 1 Ame. 20

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3nv entor Euye' ne 0. Selby attorney July 18, 1939. E. o. SELBY 2,166,694.

NOISE REDUCTION 5Y5 TEM Filed Nov. 19, 1936 2 Sheet-Sheet 2 muss mm lhwentor +3 I Eugene 0.8421129 attorney Patented July 18, 1939 greater PATENT QFFEQE NOKSE REDUCTION SYSTEM Eugene 0. Selby, Camden, N. J assignor to Radio Corporation of America, a corporation of Delaware Application November 19, 1936, Serial No. 111,688

3 Claims.

The invention relates to the reduction of noise such as that produced in radio receivers and like apparatus by impulses extraneous to the signal, and has for its principal object the provision of an improved apparatus and method or". operation whereby the undesired eii'ects of such extraneous or noise producing impulses are minimized or eliminated.

fhe problem of noise reduction is susceptible of a number of different solutions, some of which involve (l) balancing or neutralizing the eifect of the noise impulses, (2) picking up the desired signal at an antenna which is located at a distance from the radio receiver, (3) limiting or clipping the peaks of the noise impulses, (4) utilization of a special transmission circuit or (5) the use of means for automatically short circuiting, open circuiting or otherwise controlling the transmitting eificiency of the signal channel for the duration of the noise producing impulses.

The present invention involves the use of some of these features but is chiefly characterized by the use of a tuned control circuit which (1) rejects the desired signal impulses, (2) accepts and detects the noise impulses and (3) utilizes the resulting detected current to so control a grid bias potential of some stage of the signal channel as to preclude the passage of noise impulses to the final stages of this channel. A further feature of the invention is the provision of a control circuit which is eiiective to eliminate the effect of noise impulses irrespective of the relative amplitudes of the noise and signal impulses.

The invention will be better understood from the following description when considered in connection with the accompanying drawings and its scope is indicated by the appended claims.

Referring to the drawings:

Fig. l is a wiring diagram of a signal channel including a control circuit wherein the noise impulses are detected and utilized to bias an audio frequency stage below cut-oif for the duration of the noise signal,

Fig. 2 illustrates a channel which is similar to that of Fig. 1 in several respects but differs therefrom in not the signal and detected noise impulses are appl to different grids of the audio frequency stage,

Fig. 3 illustrates a channel including means for balancing out or neutralizing the detected noise impulses before they reach the audio frequency stage,

Fig. l illustrates a signal channel provided with means which operate to elhninate noise down to and including aero level of the signal,

Fig. 5 illustrates a signal channel wherein a bias potential dependent on the detected noise impulses is applied to an intermediate frequency stage instead of to an audio frequency stage, as in the channels of Figs. 1 to 3, and

Fig. 6 illustrates a similar signal channel wherein the noise impulse responsive bias is applied to a detector stage of the channel.

The channel of Fig. 1 includes an intermediate frequency stage Hi, a signal detector and noise amplifier and detector stage it which may be of the 6137 RCA Radiotron type, and an audio frequency stage !2 provided with a control grid it which is subjected to a resultant bias potential dependent on the detected signal impulses and the amplified and detected noise impulses.

The intermediate frequency stage it has its output circuit connected to the detector electrodes [4-45 through a transformer l6 and a resistor l l which is shunted by a capacitor Ill. The stage it! is also provided with diode electrodes ill-20 from which an automatic volume control current is derived for maintaining a substantially constant signal at the control point exemplified in this instance by the electron discharge device H. The maintenance of such a level at this point is advantageous from the viewpoint of segregating the noise impulses which are of relatively high amplitude and frequency.

The electron discharge device H is provided with (l) electrodes Ml5 which function to detect the signal, (2) a control grid 2! which is subjected to the signal and noise impulses, (3) an amplifier output circuit which is tuned by the transformer primary winding 22, the capacitor 23 and the inductor 24 to reject the signal impulses and accept the noise impulses, and (4) detector electrodes l525 which are connected to the transformer secondary winding 25 through a resistor 2'! which is shunted by a capacitor 28. The capacitor 23 and the winding 22 are tuned to a frequency below resonance at the signal frequency to render the reactance of the circuit 22-23 capacitive and of such magnitude as to equal the inductive reactance of the inductor 2 3 in series with it, at the signal frequency. The inductance of the inductor 24 and the capacitive reactance of the circuit 2223 provides a series resonant circuit in a filter which is responsive to the signal frequency and acts as a rejector circuit at that frequency which lies in the middle of the pass band of the signal transmission channel which may be relatively wide.

With these connections, detected signal impulses are applied to the audio frequency stage Ill grid l3 through a coupling capacitor 29 and a direct current bias potential dependent on the noise impulses is applied to the grid I3 through a resistor 33, thus biasing the stage IE to cut-off for the duration of the noise impulse.

The channel of Fig. 2 is similar to that of Fig. l in some respects but differs therefrom in that (l) the intermediate frequency stage It! is provided with detector electrodes 2fl--3l and network 3233-34--35 through which the detected signal impulses are applied to the control grid l3 of the audio frequency amplifier l2, and (2) the electron discharge device H is provided with a control grid 2i which is subjected to the signal and noise impulses, with an output circuit 2223-2426 tuned to reject the signal impulses and accept the noise impulses and with triode electrodes l--3536 which function through a resistor 30 to apply to a grid 31 of the audio frequency stage 2 a bias potential dependent on the noise impulses.

With these connections, the triode grid 35A is subjected to a potential dependent on the noise impulses and the potential developed in the triode output circuit resistors 38--39 is utilized to bias the audio frequency stage [2 to cut off for the duration of the noise impulse. As indicated, the different electron discharge devices may be respectively of 6B7, 6F? and 6A7 RCA Radiotron types.

The channel of Fig. 3 differs from that of Fig. 2 in that the noise impulses amplified by the stage H are detected by a device 40 and applied through a resistor 4| and a capacitor 42 to the terminal of the audio frequency stage control grid [3 in phase opposition to the noise component of the signal impulse applied to this terminal through capacitor 35. The noise impulses are thus neutralized or balanced out to any desired extent by adjustment of the terminal 43 of the resistor 4|.

The channel of Fig. 5 includes several features described in connection with the foregoing figures but differs in that the signal is detected and amplified in the audio frequency stage I?! which also provides a source of automatic volume control potential indicated at the terminal 43. Other differentiations from the previous figures are that (l) the signal and noise impulses are applied through parallel circuits to the control grid of the noise amplifier and detector I l and to the control grid of the intermediate stage in, (2) the amplified and detected noise impulses are applied to a separate intermediate frequency stage grid 44 for the purpose of biasing this stage to cut off for the duration of the noise impulses, and (3) a noise impulse responsive automatic volume control potential is applied through a terminal 45 to one or more of the preceding radio frequency stages.

utilized to eliminate the effect of noise impulses irrespective of the presence or absence of signal impulses. In this arrangement, the control grid 41 of a noise detector device 48 is returned to the cathode of the intermediate frequency stage In. With zero signal the grid 41 is positive and the plate current of the device 48 produces in the resistor 49 a potential whereby the audio frequency stage I 2 is biased to cut off. When signal impulses are received, the rectified current of resistor biases the grid 47 negative and the device 48 is adjusted to cut off by means of resistor 50. Under these circumstances, negative surges are cut off by the noise detector 48 and positive surges cut off the audio frequency stage for the duration of the noise impulses.

I claim as my invention:

1. The combination of means providing a signal amplifier adapted to receive and amplify signal and noise impulses in a relatively wide band of frequencies, an electron discharge device in said amplifier provided with grid electrode means for controlling the flow of signal and noise impulses, means in said amplifier for subjecting said grid electrode means to a control potential dependent on said signal, means in said amplifier including a single tuned filter circuit for rejecting the desired signal impulses and accepting and detecting the noise impulses, thereby segregating said noise impulses from the signal impulses, and means for subjecting said grid electrode to a bias potential dependent on said segregated impulses.

2. The combination of an intermediate frequency amplifier adapted to receive signal and noise impulses in a relatively wide frequency band, an electron discharge device coupled to said amplifier and provided with a pair of grid electrodes for controlling the signal flow therethrough, means for subjecting one of said grid electrodes to a control potential dependent on said signal impulses, and filter means for receiving signal and noise impulses, said filter means being tuned to a narrow frequency band within said first named band for selectively excluding signal impulses and subjecting one of said grid electrodes to a control potential dependent on said noise impulses.

3. In a radio receiving system, the combination with an intermediate frequency amplifier stage, of means providing a signal detector and a noise amplifier stage, means coupling said signal detector and noise amplifier to said intermediate frequency amplifier stage, a rectifier coupled to said noise amplifier stage, means providing a tuned filter circuit for applying signals from said noise amplifier stage to said rectifier passing a relatively wide band of frequencies on either side of the intermediate frequency and for attenuating signals at the intermediate frequency, and an additional tube in the signal channel of said receiving system having grid electrode means for receiving a controlling potential from the signal detector and from said rectifier operative to preclude the passage of noise impulses therethrough in response to controlling potentials from said rectifier.

EUGENE O. SELBY. 

